The present invention relates to a process for the separation of aqueous mixtures of arsenic acid or salts thereof from a waste mixture comprising sulfuric and arsenic acids and producing a high purity arsenic acid product.
Commonly assigned U.S. Pat. No. 5,089,241 describes a process for the conversion of hazardous hexafluoroarsenic acid or any salt thereof to arsenic acid. The resulting hazardous mixture comprises about 45 to about 85 weight percent sulfuric acid, about 5 to about 25 weight percent arsenic acid, and about 15 to about 40 weight percent water. The patent teaches that this hazardous waste may be rendered non-hazardous by the use of known methods.
Rather than generate waste for storage in a landfill, a process is needed which separates the arsenic acid from the waste mixture such that the separated arsenic acid may be reused. A process which also separates the sulfuric acid such that the separated sulfuric acid also could be reused would be extremely advantageous.
Attempts have been made to solve this problem of separating the arsenic acid or salts thereof from sulfuric acid in various media but the following problems have been encountered. L. Nan et al., "Kinetics of Reduction of Arsenic (V) to Arsenic (III) with Sulfur Dioxide in Aqueous Solution", Australas. Inst. Min. Metall. Publ. Ser. 1989, 6/89 (Non-ferrous Smelting Symp., 1989, 145-52) report that the reduction of arsenic acid to arsenious acid with sulfur dioxide in aqueous solution proceeds in two steps: the absorption of the sulfur dioxide from the gas phase into the aqueous phase and the reduction of the arsenic acid. Sulfuric acid was initially added to the arsenic acid to adjust pH. The weight percent of arsenic acid used was 1 to 14 while the weight percent of sulfuric acid used was 0.5 to 30. Most of the experiments were conducted with 0.01M or 0.1M initial arsenic acid and a small amount of sulfuric acid added to adjust the pH to about 4. The reference reports that when the initial arsenic acid was then increased to 1M, the arsenic reduction rate decreased by about 30 percent. The reference teaches that the reaction rate was considerably reduced in concentrated sulfuric acid solutions. Thus, the reference discourages the use of a starting material having higher concentrations of sulfuric and arsenic acids therein. Further, the reference teaches that the arsenious acid is the final product and not further reacted. See also K. Gritton et al., "Metal Recovery from Metallurgical Wastes", Society for Mining, Metallurgy and Exploration, Inc, Feb. 26-Mar. 1, 1990.
East German Patent 248,249-A3 dated Aug. 5, 1987 teaches a process for separating arsenic trifluoride from a product mixture containing hydrogen fluoride. The process involves reacting the mixture of arsenic trifluoride and hydrogen fluoride with arsenic trioxide and sulfuric acid wherein the hydrogen fluoride reacts with the arsenic trioxide to form arsenic trifluoride. The reference teaches that the resulting arsenic trifluoride may be used as an implantation gas in the microelectronics industry and does not teach that the arsenic trifluoride is further reacted.
U.S. Pat. No. 4,891,207 teaches the oxidation of arsenic trioxide with hydrogen peroxide to form arsenic acid. The reference does not teach how to separate arsenic acid from a mixture containing sulfuric acid.
As such, the need exists in the art for a process for separating arsenic acid from an aqueous waste mixture comprising sulfuric and arsenic acids.